Bark feeding system



United States Patent O 3,402,684 BARK FEEDING SYSTEM Joseph Gradischer, Bradfordwoods, Pa., and Thomas J. Smith, Bloomfield, Coun., assgnors to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed .lune 26, 1967, Ser. No. 648,813 Claims priority, application Japan, Sept. 8, 1966, 41/59,383 8 Claims. (Cl. 110-102) ABSTRACT F THE DISCLOSURE A system for conveying bark or material of similar character and feeding it to a furnace partially by conveying means using hot air or hot gases with a portion of the path over which the bark travels being maintained the atmospheric pressure. The bark is transferred from the atmospheric pressure portion to the subtatmospheric pressure portion by means of a gate which comprises a pair of large diameter air-lled rotating rollers and sealing means fully occupying the cross section of the bark conveying path, the rollers having a soft, yielding surface maintained in mutual contact to form a seal while the bark passes between the rotating rollers from the atmospheric pressure portion of the conveying path to the subatmospheric pressure portion.

Background of the invention The invention relates to a system for conveying and feeding bark, bagasse, or other cellulose fuel of similar character and has special reference to improved feeder facilities by the aid of which such bark and other woodtype fuel can be .successfully transported partially by air or gas to a furnace for burning and for the generation of steam or other purposes.

The three major problems encountered in suspension firing of bark in furnaces are the uniform feeding of the bark to the bark transport fan, the leakage of cold air into the bark conveying and firing system, and the control `of the hot air supplied to the bark transport fans. The principal cause of these problems in feeding bark is the fact that hogged bark is a very diicnlt material to handle, primarily because it contains stringers or long threads of bark that tend to tie the pieces, chips, or slivers of bark together. In addition, these stringers have a tendency to wrap themselves around any rotating parts having projections or diameters less than the length of the stringersf Summary of the invention The invention resides in an improved bark feeding facility and has as its objects uniform and trouble-free feeding `of the bark to the bark transport fans. This is accomplished by the employment of a pair of large diameter feed rollers preferably being air lilled and having a soft, yielding surface which permits the bark to pass between the rollers uniformly and without danger of entanglement. At the same time, because of the yielding character of the rotating surface, a satisfactory seal is maintained between the atmospheric and subatmospheric pressure regions of the bark conveying path.

Brief description of the drawing Other objects and advantages of the invention will become apparent from the following description of an illustrative embodiment thereof when taken in conjunction with the accompanying drawing wherein:

FIGURE 1 is a diagrammatic representation of a bark feeding system as heretofore used in the art;

FIGURE 2 is a diagrammatic representation of a bark 3,402,684 Patented Sept. 24, 1968 ICC feeding system equipped with the inventive facilities herein disclosed; and

FIGURE 3 is an isometric view showing the duct portion containing the large diameter feed rollers and the sealing strips.

Description of the preferred embodiment In the drawing, like reference characters are used throughout to designate like elements. Referring now to FIG. 1, an existing bark feeding system as presently known in the art will first be described to facilitate understanding of the invention and of the advantages afforded by its use. The bark received from a storage area (not shown) is hogged in a hogger 10 and delivered by way of a conveyor belt 12 to a live-bottom storage bin 14 from whence the bark is fed into a chute 16. This chute terminates in feeder 18 which is a drum provided with a great number of projections upon its periphery and is made to revolve at a predetermined speed which determines the quantity of bark per hour delivered by Way of chute 20 to the top of cyclone mixer 21. Hot air or gas is admitted to cyclone mixer 21 through duct 22 and damper 23. After thorough mixing which serves to remove surface moisture from the bark, the air and bark mixture passes to bark transport fan 24 which delivers the air-suspended bark to burners 26 by way of fuel pipe 27 for burning in furnace 28.

In the apparatus described above, a negative pressure must be maintained in the portion of the bark travel path between the bark feeder 18 and the inlet to fan 24 in order to prevent bark powder and hot air from blowing out into the surrounding area. For this reason, an air seal must be established to prevent cold air from leaking into the system from the bark feeding end. An attempt is made to provide such a seal by maintaining a head of bark of about 10 feet in chute 16.

The difficulties encountered in the above-described system are caused primarily by the live-bottom storage bin 14, the air sealing chute 16, and the bark feeder 18. The live-bottom storage bin is subject to frequent plugging due to the mechanical bottom. The tall column of bark maintained in chute 16 intended to form an air seal performs this function only in a haphazard manner, still permitting undesirable leakage of cold air into the system. In addition, this column of bark in chute 16 has an inherent tendency to bridge and to plug the inlet to feeder 18. Feeder 18, being a rotating drum with projections on its outer surface, has a tendency to become bound with stringers and plug. All of the above diculties cause frequent shutdown of the bark firing system which results in an inefficient operation, costly maintenance and large investment and income losses.

Our improved bark feeding system as illustrated in FIGS. 2 and 3 overcomes the above problems and will now be described in detail. First, the live-bottom storage bin 14 together with motor drives and controls, and air seal chute 16 are completely eliminated. This does away with a frequent source of plugging in live-bottom bin 14 and bridging and plugging in duct 16. In addition, elimination of these elements from the system will result in a saving in head room, building space and supporting steel. Second, the bark feeder 18 is replaced by a device 30, later to be described herein in detail, which assumes the function of the air seal of former chute 16 and the function of former bark feeder 18, without being subject to the disadvantages of these elements. Thus the bark from the hogger 10 and conveyor belt 12 is received by a feeder and air lock device 30 which delivers the bark directly into chute 20, and cyclone mixer 21.

Referring now to FIG. 3, showing an enlarged view of feeder and air lock device 30 which comprises a housing 31 of generally rectangular configuration open at the top for receiving the bark from conveyor 12, and having a hopper bottom 32 connected to chute 20 leading to a cyclone mixer 21. Two cylindrical rollers 34 and 35 are closely mounted within the housing 31 between and on opposing side walls 36 and 37. They are rotated in opposite directions, as indicated by the arrows, by means of a motor drive 38 and suitable pulleys 39. In accordance with the invention, drums or rollers 34 and 35 are of large diameter, such as 3 feet, exceeding the possible length of stringers, are fabricated of a rubber fabric or a rubberlike fabric, and are hollow and filled with air at relatively low pressure. The peripheral surfaces of these rollers are soft and yielding and in intimate contact one with the other. As they rotate, the outer upper surfaces converge and grip and pull downwardly the pieces of bark which fall on the rollers from the conveyor belt 12. Due to the soft, yielding character of the rubber fabric and the internal low pressure, the surface tends to close around the bark pieces and thereby pre- Vents substantial leakage of cold air into the system. To further eliminate such leakage, a rubber seal 40 is provided 'between the Surface of drum 34 and side wall 41 and a rubber seal 43 between the surface of drum 35 and side wall 44 extending throughout the entire length of drums 34 and 35. This length, which preferably is in the neighborhood of feet, is such that the drum fits snugly between the side Walls 36 and 37 The position of hot -air damper 23 is determined in accordance with the invention by the temperature of the air and fuel mixture entering the furnace by way of burners 26. For this purpose a temperature measuring device 46 is provided in fuel pipe 27, which device transmits control impulses representing temperature variations -to a controller 48 for the purpose of regulating damper 23. Thus if the temperature of the fuel and air mixture increases above a predetermined desired amount, the increase causes impulses to be transmitted to controller 48 which will close damper 23 a certain amount. A decrease of temperature, on the other hand, will tend to open damper 23 to permit more hot air to enter cyclone mixer 21. Such variations in temperature of the air and fuel mixture may be caused by variations in moisture content of the bark entering the system, or by variations in hot air temperature.

From the above it can readily be appreciated that our improved bark conveying system overcomes the three earlier mentioned major problems in the firing of bark or other similar fuel which plagued earlier installations. Thus uniform feeding of bark to the transport fan 24 is now assured, since the possibility of bridging of the bark in an air seal shaft or chute 16 and the resulting plugging and interruption of flow has now been completely eliminated. Furthermore, stoppage of lbark delivery due to entanglement of the rotating parts of a liveebottom bin 14 and bark feeder 18 with stringers is now practically prevented by the elimination of bin 14 and by the use of feed rollers 34, 35 having large diameters in place of bark feeder 18. Cold air leakage into the system, another serious problem, is now also greatly alleviated by abandonment of the Iair seal column 16 and by the use of air-lled feed rollers 34, 35 which have a soft and yielding working surface. Furthermore, hot air control is greatly improved by making the quantity of hot air entering the system now depending upon the temperature of the air and fuel mixture entering the burners 26.

While the invention has been described herein in connection with the burning of bark, our improved fuel feeding system can with equal benefit be used in connection with other type fuels having similar physical characteristics, such as bagasse or other fibrous or stringy material.

We claim:

1. A system for feeding a fibrous and stringy material to a furnace by mechanical and by gas-supported means, including a material conveying path having a first zone subject to atmospheric pressure and a second zone subject to subatmospheric pressure connected in series with said first zone, and means for conveying the material from said first zone to said second zone, the improvement comprising a rotary air lock feeder interposed between said zones and having a pair of large diameter gas filled feed rollers with soft yielding cylindrical surfaces, means for rotating said rollers in opposite directions, means for urging said rollers into mutual gas sealing contact, `said rollers being adapted for receiving said material and passing it between said -rollers from said first zone to said second zone.

2. A system as defined in claim 1 wherein sealing means are provided at the outer circumferential periphery of said rollers to seal said first pressure zone from said second pessure zone.

3. A system as defined in claim 1 having means for introducing hot gas into said subatmospheric pressure zone for conveying said material in suspension.

4. A system as defined in claim 3 wherein said subatmospheric pressure zone includes -a cyclone means for mixing said hot gas and material to dry the latter.

5. A system as defined in claim 4 wherein said subatmospheric pressure zone vincludes a material transport fan for creating said subatmospheric pressure and for conveying said hot gas suspended material to said furnace for burning.

6. A system as defined in claim 5 having means for measuring the temperature of said gas-suspended material entering said furnace, and having means for increasing or decreasing the quantity of said hot gases in response to a decrease or increase, respectively, of the temperature of the gas suspended material entering said furnace.

7. A rotary `air lock feeder mounted in -a duct portion for feeding fibrous stringy material of variable lengths from a first pressure zone to a second pressure zone, the improvement comprising a pair of -large diameter rollers, means fo maintaining said rollers in circumferential intimate contact, means for rotating said rollers in opposite directions, said rollers having a cylindrical hollow body filled with an expandable medium and being made of a soft yielding material to form a seal along the common circumferential surface, the diameter of said rollers being larger than the lengths of said fibrous stringy material to prevent entanglement with said material when passing between said rollers from said first to said second zone.

8. A rotary air lock feeder as defined in claim 7 having the additional improvement of sealing strips mounted on opposing interior sides of said duct portion in sliding, yielding and sealing contact with the circumferential periphery of said rotating rollers.

References Cited UNITED STATES PATENTS 1,065,924 7/ 1913 Dixon 30262 X 3,064,592 11/1962 Eberhardt 110-7 3,274,961 9/1966 Wiechard et al. 110--56y JAMES W. WESTHAVER, Primary Examiner. 

